Microblog.pub sometimes writes error log as follows:
```
[ERROR] cache not available for (Icon's Path)/50/Kind.ACTOR_ICON
```
(Maybe) for the sake of caching actor's icon, `_cache_actor_icon()`
in `core/activitypub.py` runs `Tasks.cache_actor_icon()`.
However, `cache_actor_icon()` in `core/tasks.py` does not run
`/tasks/cache_actor_icon`.